Servo currently builds on Mac OS X and Linux. Since the Rust language is currently in flux, you will need the bleeding-edge (Git master) version of the Rust compiler.
On OS X:
brew install https://raw.github.com/Homebrew/homebrew-versions/master/autoconf213.rb
brew install sdl cairo
On Debian-based Linuxes:
sudo apt-get install libsdl1.2-dev libcairo2-dev libpango1.0-dev autoconf2.13 freeglut3-dev
git clone git://github.com/mozilla/servo.git
cd servo
git submodule init
git submodule update
./autogen.sh
mkdir -p build && cd build
../configure
make check && make
./servo ../src/test/test.html